Gut reaction A US study has found weight loss surgery appears to change the body's metabolism in a way that dieting alone cannot, and may explain why diabetes often disappears before much weight is lost.

Weight loss surgery is becoming increasingly popular as obese people struggle to lose weight and avoid the health complications that accompany the extra weight - including diabetes, heart disease, joint pain and some cancers.

In research conducted at Columbia University in New York and Duke University in North Carolina, researchers studied two small groups of severely obese diabetic patients who either had gastric bypass surgery or went on strict diets.

Both groups lost about 10 kilograms.

For the study, the teams measured metabolites - chemical by-products of foods in the body.

"What we caught is a very clear difference between bariatric surgery and dietary intervention."

He says patients in the surgery group had lower levels of molecules known as branch chain amino acids, which are known to be associated with insulin resistance.

"These dropped much more precipitously in people having bariatric surgery than people having the dietary intervention," he says.

People in the gastric bypass arm of the study underwent a surgery known as Roux-en-Y, in which doctors surgically reduce the size of the stomach to prevent people from eating too.

Newgard says it is not clear why reducing stomach size might have this effect, but it is clear that bariatric surgery results in significant metabolic changes.

The team is now looking to discover ways to develop drugs that could replicate this effect.

Newgard says the results might not apply to Allergan's Lap-Band weight-loss device, in which doctors insert an adjustable silicone band around the upper part of the stomach but do not surgically reduce the size of the stomach.

Insights into the human gut

Associate Professor John Dixon, an obesity researcher at the Baker IDI Heart and Diabetes Institute in Melbourne, says this latest research provides greater insights in the role the human gut plays in regulating insulin.

"The people who had the gastric bypass had a greater output of the hormone GLP-1 and that stimulated insulin secretion ... which had a greater effect on improving glucose tolerance in that group," says Dixon.

GLP-1 is secreted by cells in the lower gut, slowing the absorption of nutrients into the blood stream and increases the secretion of insulin.

According to Dixon gastric bypass is a risky procedure with a mortality rate eight to ten times than lap-band surgery, but is still used in cases where rapid weight-loss is needed.

He says studies such as this allows researchers to better understand the "secrets that the gut has in controlling energy balance" and could lead to the development of pharmaceutical-based therapies that are used with, or without, surgery to treat obesity and type 2 diabetes.